Heat transfer tube assembly

ABSTRACT

A heat transfer tube assembly (heat pipe) using lithium as its working fluid is provided with 5-500 p.p.m. of an oxygenstabilizing agent in the refractory alloy of the wall material to prevent oxygen corrosion at operating conditions.

United States Patent Gunther Kraft Kronberg, Taunus, Germany;

Manfred Potzschke, Frankfurt am Main, Germany; Claus Busse, Laveno, Italy;

[ 72] Inventors Franz Geiger, Arolo di beggiuno, Italy [21] Appl. No. 826,956

[22] Filed May 22, 1969 [45] Patented Aug. 31, 1971 [73] Assignee M Aktieugesellschaft Frankfurtam Main, Germany [32] Priority May 25, 1968 [33] Germany [54] HEAT TRANSFER TUBE ASSEMBLY 3 Claims, 1 Drawing Fig.

[52] U.S.Cl 165/105,

[51] Int. Cl ..F28d 15/00, F28f 19/00 [50] Field of Search 165/105, 134

[56] References Cited OTHER REFERENCES Deverall et a], JE High Thermal Conductance Devices Utilizing The Boiling of Lithium or Silver, Los Alamos Scientific Laboratory, Los Alamos, New Mexico, 4/1965, 37 pgs.

Primary Examiner-Albert W. Davis, Jr. Attorney-Burgess, Dinklage & Sprung ABSTRACT: A heat transfer tube assembly (heat pipe) using lithium as its working fluid is provided with 5-500 ppm. of an oxygen-stabilizing agent in the refractory alloy of the wall material to prevent oxygen corrosion at operating conditions.

HEAT

HEAT

PATENTEB M183] 1911 3,602,297

HEAT TRANSFER TUBE ASSEMBLY The present invention relates to the use of refractory metals or their alloys, which contain oxygen-stabilizing addition elements, as wall material for heat transfer tubes which carry molten metallic lithium as a heat transfer fluid.

Heat transfer tubes are tubes which are hermetically sealed on all sides and which contain a capillary system formed on the inside surfaces by ribs and grooves and which are partly filled with a heat transfer fluid. When one end of such tube is heated, the heat transfer fluid will form a liquid-vapor cycle between the heating zone and a remote cooling zone. The heat transfer fluid transfers heat in that direction with such low losses that the temperature of the entire heat transfer tube may be considered virtually constant.

Such heat transfer tubes are often used to transfer heat to the emitter and to transfer heat from the collector of thermionic energy-converting devices. Operating temperatures of about l400-l600 C. are required for a cooperation of such tubes with the emitters of thermionic converters so that the selection of the heat transfer fluid and the wall material of the heat transfer tube is restricted. Refractory metals, such as columbium, zirconium, molybdenum, tungsten, tantalum, or their alloys, such as columbium and 1 percent zirconium, tantalum and 5 percent tungsten, have been used as wall materials. Bismuth, lead, thallium, barium and the alkali metals are known as metallic heat transfer fluids. The known wall materials such as molybdenum or tungsten have proved unsatisfactory in many cases because they are too brittle. Tube walls made from columbium alloys containing 1 percent zirconium or of tantalum have an unsatisfactory life of a few days until the wall is burnt through when the heat transfer tubes are operated with lithium. The corrosion process is critically influenced by the oxygen content of the wall material of high-temperature heat transfertubes. This corrosion process involves a dissolving of the oxygen out of the wall material by molten lithium, which flows along the inside wall under the operating conditions. The molten lithium transfers the oxygen into the heating zone, where the oxygen is deposited in the form of oxides of lithium and of the metal of the wall material. The'corrosion can be controlled only if the tube material contains less than 1 p.p.m. oxygen. It has hardly been possible before to meet these requirements in commercial operation. Finally, a commercially available tantalum has a small content of yttrium and is known as so-called SGS tantalum or grain-stabilized tantalum.

It is an object of the invention to provide for heat transfer tubes operated with lithium a wall material which has an excellent, high resistance to corrosion,

To accomplish this object, the invention teaches to use oxygen-stabilized refractory metals. In accordance therewith, the invention resides in the use of refractory metals or their alloys and 5-500 p.p.m. oxygen-stabilizing elements as wall material for heat transfer tubes containing liquid metallic lithium.

An embodiment of a heat transfer tube assembly in accordance with the invention is shown by way of example in a longitudinal sectional view in the drawing.

The heat transfer tube assembly comprises a horizontally disposed tube 1, which is sealed at opposite ends by plugs 2 and 3, respectively. The tube 1 is formed on its inside surface with a plurality of axial grooves 4 which are circumferentially spaced apart and may be covered with a screen. The screen is not shown in the drawing for the sake of clarity.

The tube 1 contains some metallic lithium which at operating temperature fills the grooves 4, whereas lithium vapor fills the inner part of the tube.

The tube 1 has a heating zone 5 adapted to be supplied with heat and a cooling zone 6, from which heat can be withdrawn.

The tube 1 and any screen therein consists of a refractory alloy which contains 5-500 p.p.m. of an oxygen-stabilizing element, A preferred material for tube 1 is known as $68 tantalum, a commercially available, grain-stabilizing tantalum alloy which contains a small percentage of yttrium. Alternatively, the tube 1 may consist of a columbium alloy containing 1 percent zirconium.

Generally, tantalum which contains yttrium or rare earth element metals, such as cerium or lanthanum, has proved satisfactory, e.g., as a wall material in lithium-operated heattransfer tubes. The tantalum suitably contains said additional elements in amounts of 5-500 p.p.m. The use of tantalum which contains 5-500 p.p.m. yttrium as a wall material in lithium-operated heat-transfer tubes has proved particularly suitable. Tantalum alloys, such as tantalum and 5 percent tungsten as well as additions of oxygen-stabilizing elements in amounts of 5-500 p.p.m. e.g., yttrium or rare earth elements metals, may also be used as a wall material according to the invention. Columbium or its alloys, such as columbium and 1 percent zir conium as well as additions of oxygen-stabilizing elements in amounts of 5-500 p.p.m. are also suitable as a wall material according to the invention. The use of refractory metals which contain oxygen-stabilizing elements as wall material in lithium-operated heat-transfer tubes involves advantages. These additions, such as yttrium or rare earth element metals, reduce the oxygen-diffusion rate of the oxygen in the wall material to such an extent that oxygen is no longer presented to lithium, which as a result of the continuous evaporation-condensation cycle has an extremely low oxygen content so that it has a high free linkage enthalpy for oxygen. A heat transfer tube consisting of tantalum and 5-l00 p.p.m. yttriums has been successfully used according to the invention at 1600 C. for more than 1000 hours without damage to the capillary structure.

What we claim is:

1. A heat transfer tube assembly, which comprises a heat transfer tube consisting of a refractory alloy containing 5-500 p.p.m.of an oxygen-stabilizing agent, means sealing said tube at both ends, and a body of metallic lithium partly filling said tube.

2. A heat transfer tube assembly as set forth in claim 1, in which said oxygen-stabilizing agent is selected from the class consisting of yttrium and the rare earth elements.

3. A heat transfer tube assembly as set forth in claim 2, in which said refractory alloy is selected from the class consisting of alloys of tantalum and columbium. 

1. A heat transfer tube assembly, which comprises a heat transfer tube consisting of a refractory alloy containing 5-500 p.p.m. of an oxygen-stabilizing agent, means sealing said tube at both ends, and a body of metallic lithium partly filling said tube.
 2. A heat transfer tube assembly as set forth in claim 1, in which said oxygen-stabilizing agent is selected from the class consisting of yttrium and the rare earth elements.
 3. A heat transfer tube assembly as set forth in claim 2, in which said refractory alloy is selected from the class consisting of alloys of tantalum and columbium. 